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Abstract— Bacterial suspensions in a phosphate buffer were irradiated at 364, 404, 434, 454, 484, 520, 560. 570, 599, 619, 630, 633, 640, 693, 730 and 750 nm, incubated in Hottinguer broth, and assayed for viability by the standard surface plating technique. When irradiated at 404, 454, 560, 619, and 750 nm, the cells started dividing more rapidly, the growth stimulation effect reaching170–180% 60 min after irradiation. The shape of the action spectrum and the reduced‐minus‐oxidized difference spectrum of the bacterial suspensions suggest that respiratory chain components of the bacterium such as dehydrogenases and cyt d might be the photoacceptors involved. The growth stimulation effect is believed to be due to the ability of the irradiation‐activated respiratory chain to produce the necessary pH gradient more rapidly, thus enabling the cells to start dividing earlier.